1. Field of the Invention
The invention relates to the field of semiconductor and dielectric optical components used for optical transmission or optical digital data processing. It pertains especially to all optical components comprising active and/or passive waveguides and also to single and multimode fibers, for example components such as semiconductor lasers, semiconductor amplifiers, modulators, and wavelength filters, to name a few. There is a great deal of ongoing research and development effort to reduce the cost of optical modules while maintaining the minimum loss of optical power. A major component of the fabrication costs usually arises from the joining of such components to optical fiber, as for example when it is necessary to couple optical modes of very different sizes. Thus, when a laser and a flat-end single-mode optical fiber are joined together, the optical mode of the laser or a high-index contrast (HC) guide spot size with a diameter of, for example 1-2 um, has to be coupled with the optical mode of a single mode (SM) optical fiber whose diameter is far greater, for example in the range of 5-10 um.
To enable the coupling of these optical modes of very different sizes, spot-size converters or mode transformers are made in order to increase the size of the mode at the output of the optical component and make its profile compatible with that of the mode guided in the optical fiber. The reverse can also be accomplished to decrease the size of the mode from a single mode (SM) fiber to a high-index contrast (HC) waveguide. However, this mode matching must be done while preserving the performance characteristics of the component.
For instance, in connecting a SM fiber having a mode spot size of 8 μm, to a HC waveguide having a spot size of 1.5 μm, over 90% of the power is lost. Such loss is intolerable in optical communication systems. FIG. 1 shows the relative sizes of the modes (field patterns) of a conventional SM fiber (100), and that of a HC waveguide. (110). The SM fiber spot size is typically 5-10 μm which is as much as an order of magnitude greater than that of an HC waveguide—typically 1-2 μm.
When transforming the modes between two waveguides with different refractive index and/or core sizes, high coupling loss arises due to the difference in mode size, shape, and mode velocity. For example, the index difference and the mode size of a fiber optic waveguide are different than those of a high index difference planar waveguide, resulting in high coupling loss when the fiber optic waveguide and the high index difference planar waveguide are directly coupled.
A channel waveguide is a dielectric waveguide whose core is surrounded by a cladding that is comprised of materials with refractive indices lower than that of the core, and wherein the peak optical intensity resides in the core. Waveguides in general can be defined by other geometries as well. A high index contrast (HC) waveguide is defined as one where the core cladding index contrast is larger than that of a typical single mode fiber (that is, larger than approximately 1%). HC waveguides also typically have mode field diameters that are smaller than that of a single mode fiber by a factor of two.
In optical components, it is essential to have low coupling loss when attaching a fiber to a microchip. To efficiently couple two waveguides with very different dimensions and therefore two different spot sizes, some sort of mode transformer is required. As shown in FIG. 2, the mode transformer essentially acts as a funnel, necking down a wide area spot from the SM fiber (200) to a small area spot associated with the HC waveguide (220). FIG. 2 depicts the functionality of such a mode transformer (210)
A mode transformer between two different waveguides is an essential part of an optical
system where the lightwave (mode) from one optical component is coupled into another component. In optical communication, a mode transformer between an optical fiber waveguide and a high index difference (difference in the refractive indices of core and cladding) planar waveguide is crucial for successful implementation of planar lightwave circuits (PLC) in fiber communication. Therefore, developing an efficient mode transformer between two waveguides has continued to be a subject of intense research.
In addition, the core index of the fiber optic waveguide is lower than that of the high index difference planar waveguide causing a mode velocity difference between two waveguides. As will be detailed in the Detailed Description section, when such a change in mode properties takes place too quickly, high power loss arises.
2. Prior Art
There have been several other approaches to achieve efficient mode coupling between two waveguides with different index difference, including mode transformation by tapering the dimension of high index difference waveguides. Mode transformation by a taper has been shown in various publications. Over the tapering region of the high index difference waveguide, the thickness or width of the waveguide core is gradually tapered down from that of the normal guiding region to a lower thickness or width. As the mode travels from the normal guiding region of the high index difference waveguide into the tapering region, the mode experiences decreasing amount of the core material. The fraction of the mode field distribution that exists outside the core material increases, changing the mode size. The index of the waveguide that the mode experiences is effectively changed by the presence of the taper. In other words, the “effective index” is gradually changed by the taper. By gradually changing the effective index from that of the low index waveguide to that of the high index difference waveguide, the mode coupling can be achieved between two waveguides without high loss. The method to determine the effective index is described in “The Handbook of Photonics”, Boca Raton, Fla. CRC Press, 532-535 (1996) by M. Gupta.
T. Brenner et. al. (“Integrated optical modeshape adapters in InGaAsP/InP for efficient fiber-to-waveguide coupling,” IEEE Photonics Tech. Lett. Vol. 5, No. 9, 1993) show a mode transformer using a vertically tapered high contrast waveguide. Vertical tapering uses special etching techniques that are not well controlled and therefore difficult to manufacture. Also the vertical taper shape cannot be arbitrarily specified, but is more a function of etching characteristics, rather than design. The mode size propagating in the tapered region increases due to the reduction of the effective index, and thus the reduction of the effective index difference. The publication indicates the gradual mode transformation occurring in one waveguide due to the presence of a taper.
U.S. Pat. No. 5,199,092, issued to Stegmueller et al. shows the coupling of modes between two different waveguides: one broad and one narrow. The two waveguides run parallel to one another and are superimposed with each other to provide a superimposing waveguide guidance. During the superimposed waveguide guidance, one of the two waveguides is tapered down in vertical dimension, while the other waveguide dimension is kept constant. The role of the tapered waveguide is to provide a gradual effective index change, and thus mode transformation, in the same manner as the cases cited in journal publications including that by Brenner et al. The difference is the superimposition of the narrow waveguide, providing waveguiding in the broad waveguide once the narrow waveguide is completely terminated by the vertical taper. The broad waveguide is surrounding the narrow waveguide over the whole waveguiding distance. The presence of the broad waveguide helps guiding the mode once the mode transformation is complete.
In addition to single taper devices described above, dual tapers are used in mode transformation between two different waveguides. IEEE Photonic Technology Letters, Vol. 7, No. 5, May 1995 by Zengerle et al., reports a mode transformer having two channel waveguides, each with a taper, one sitting on top of the other. Electronics Letters, Vol. 29, No. 4, February 1993 by Schwander et al., reports a mode transformer having two rib waveguides, each with a taper, a portion of one embedded within the other. Both of the rib waveguides used in the art are weakly guiding. This is not a suitable method for mode transformation to or from a high index difference waveguide.
Y. Shani et. al. (“Efficient coupling of a semiconductor laser to an optical fiber by means of a tapered waveguide on silicon”, Appl. Phys. Lett. vol. 55, No. 23, 1989.) describe a mode transformer using a taper embedded within a second larger waveguide. Their taper is however adiabatic. In that case the taper was wedge shaped (linearly tapered) and very long in order to make use of the adiabatic mechanism. The taper is required to
also come down to a sharp point, which makes it almost impossible to perform in a lithographic process.
B. M. A. Rahman et. al. (“Improved laser-fiber coupling by using spot-size transformer”, IEEE Photonics Technology Lett. Vol. 8, No. 4, 1996) describe a mode transformer using two synchronously coupled waveguides, where one guide is a smaller high contrast guide and the other has a spot size approximating the size of a fiber mode. Their mode transformer does not use any mode evolution process, and the coupling is not terminated, causing coupling to periodically transfer between the two guides indefinitely.
G. A. Vawter et. al. (“Tapered rib adiabatic following fiber couplers in etched GaAs materials for monolithic spot-size transformation,” IEEE J. Selected Topics Quantum Electronics, Vol. 3, No. 6, 1997) show an adiabatic coupler from one waveguide to another where the high contrast waveguide is on top of the larger fiber-matched waveguide.
Variations of the above types of mode transformers can also be found in several review articles, including that by I. Moerman et. al. (A review of fabrication technologies for the monolithic integration of tapers with III-V semiconductor devices, “IEEE J. of Selected Topics Quantum Electronics,” Vo. 3, No. 6, 1997) which summarizes primarily dual type waveguide tapering.
In U.S. Pat. No. 6,253,009 entitled “SEMICONDUCTOR OPTICAL COMPONENT COMPRISING A SPOT-SIZE CONVERTER”, the invention relates more particularly to a semiconductor optical component, comprising an active waveguide and a passive waveguide that are superimposed and buried in a sheathing layer, wherein the component comprises successively: a damped coupling section in which the width of the active waveguide tapers down and the width of the passive waveguide increases, and a mode expansion section comprising only the passive waveguide whose width tapers down. According to another characteristic of the invention, the component furthermore comprises a transition section positioned between the damped coupling section and the mode expansion section, in which the width of the active waveguide tapers down to 0 um. The invention enables the making of an optical component comprising an integrated spot-size converter, wherein the optical mode is chiefly deconfined in the passive guide so much so that the current threshold and the efficiency of the component are not affected. The passive and active waveguides are not aligned but they are coupled vertically, so much so that the problems related to alignment are prevented. Furthermore, the two types of active and passive waveguide may be optimized separately.
In U.S. Pat. No. 6,130,969 entitled “HIGH EFFICIENCY CHANNEL DROP FILTER” a highly efficient channel drop filter employs a coupling element including a resonator-system between two waveguides, which contains at least two resonant modes. The resonator-system includes one or more interacting resonant cavities which in addition to being coupled to the waveguides, can also be coupled directly among themselves and indirectly among themselves via the waveguides. Each component of the coupling element can be configured or adjusted individually. The geometry and/or dielectric constant/refractive index of the resonator-system are configured so that the frequencies and decay rates of the resonant modes are made to be substantially the same. The filter can achieve 100% signal transfer between the waveguides at certain frequencies, while completely prohibiting signal transfer at other frequencies. In the invention shown, the filter is configured with photonic crystals. In accordance with alternative embodiments of the invention, there are provided channel drop filter devices with flat-top and straight-sidewall lineshape characteristics. These lineshape characteristics are realized by using several resonance to couple the waveguides, and by designing the relative position with respect to one another.
In U.S. Pat. No. 5,682,401 entitled “RESONANT MICROCATIVIES EMPLOYING ONE-DIMENSIONALLY PERIODIC DIELECTRIC WAVEGUIDES” the invention provides a resonant microcavity which includes a periodic waveguide, and a local defect in the periodic dielectric waveguide which accommodates spacial confinement of radiation generated within the waveguide around the defect. The inventive concept also provides a method of enhancing radiation confinement within a resonant microcavity and minimizing radiation losses into an associated substrate, the microcavity configured within a periodic confinement, the method including the step of increasing the refractive index contrast between the microcavity and the substrate.
In U.S. Pat. No. 5,229,883 entitled “HYBRID BINARY OPTICS COLLIMATION FILL OPTICS” the invention relates generally to means for collimating, aberration correcting, and angularly aligning the output of a diode laser array, a more particularly to a combination of a cylindrical lens and a pair of binary optical elements which are optimized to collimate, aberration correct, and align the individual diodes or a diode laser array such that each individual diode fills its aperture. Here, a cylindrical lens and a binary optical element for collimating with low optical aberrations provides an asymmetrically diverging input wavefront. The binary optical element is formed on a planar substrate on which a binary optical diffraction pattern is etched on the front surface thereof. The binary optical diffraction pattern is designed such that each ray of light from the diverging input light source will travel the same optical path length or vary from that optical path length by an integer multiple of the wavelength of the light traveling from its source to its exit from the front surface of the binary optical element. A beam angle alignment element is also provided, to be utilized in conjunction with a cylindrical lens and the binary optical element, for correcting angular misalignments of diode lasers whose output wavefront has an optical axis which is either above or below the plane in which the active region is formed. The beam angle alignment element is also a planar substrate on which a binary optic diffraction pattern is etched. The binary optic diffraction pattern of the beam angle alignment elements diffracts the wavefront exiting from the binary optic element so as to align the wavefront about its optical axis. The cylindrical lens, the binary optical element, as well as the beam angle alignment element may be used in conjunction with a single diode lasing element, as well as a one dimensional or a two dimensional laser array.
In U.S. Pat. No. 6,198,860 entitled “OPTICAL WAVEGUIDE CROSSINGS”, the invention relates to the field of optical waveguide crossings. In constructing integrated optical circuits, space constraints and the desire to operate on multiple input waveguides often necessitate waveguide crossings. It is crucial that the crossings be as efficient as possible. A typical application is optical switching, where a large number of inputs are directed to as many outputs, and crossing is necessary in order for each input to connect to every output. Simplicity of fabrication on small length scales means that the waveguides must actually intersect, and cannot simply pass over one another. Any additional three-dimensional structure adds considerable manufacturing difficulty.
the invention includes an optical waveguide structure, a first waveguide, a second waveguide that intersects with the first waveguide, and a photonic crystal resonator system at the intersection of the first and second waveguides. In accordance with another embodiment there is provided an optical waveguide crossing structure including a first waveguide that propagates signals in a first direction, a second waveguide that intersects with the first waveguide and propagates signals in a second direction, and a photonic crystal crossing region at the intersection of the first and second waveguides that prevents crosstalk between the signals of the first and second waveguides. In accordance with another embodiment of the invention there is provided an optical waveguide structure including a first waveguide, a second waveguide, and a resonator system at the intersection of the first and second waveguides, the intersection possessing a first minor plane that is parallel to the first waveguide, the resonator system supporting a first resonant mode that includes different symmetry with guided modes in the first waveguide with respect to the first mirror plane, the resonator system substantially reduces crosstalk from the second waveguide to said first wave.
Additional approaches have been envisaged to the deconfining of the optical mode of a component. Another method known as butt coupling that enables the coupling of a passive waveguide with an active waveguide is very common today. This method consists, in a first stage, in achieving the growth, on a substrate, of a first layer constituting the active waveguide formed for example by a quaternary material and in burying this layer in a sheathing layer constituted, for example by InP. These two layers are then etched locally according to a standard etching method on a zone reserved for the integration of a passive type of waveguide. An epitaxial regrowth operation is performed to make this passive waveguide. For this purpose, a layer of quaternary material capable of acting as the passive waveguide, is deposited on the substrate in the zone that is locally etched beforehand. Then it is buried in a sheathing layer made of an InP for example. The structure of the active waveguide is different from that of the passive waveguide. The coupling interface between the two types of waveguides is called a butt joint. Furthermore, to enable the deconfining of the optical mode, the thickness of the passive guide tapers evenly all along the passive section.
This method of manufacture is fully mastered at the present time. However, it requires an additional step of etching and epitaxially regrowth, thus giving rise to an increase in the cost price of the component. Furthermore, for aligning the active and passive guides, the alignment tolerance values remain low. Although the technique of butt coupling is well mastered, it remains a difficult and extremely important step. This method is relatively complex to implement and entails costs that are still high.
One other method known as the method of selective epitaxial growth, has been considered. In this method, the composition of a waveguide is made to vary continuously, to make it gradually go from an active waveguide state to a passive waveguide state. The selective growth of the material constituting the waveguide is achieved on a substrate by the use of two dielectric masks, made of silica (SiO2) or silicon nitride (Si3N4) for example, placed side by side. The species under the growth do not get deposited on these masks, and a phenomenon of diffusion of species under growth is created. The shape of the masks is determined so that the phenomenon of diffusion of the species is pronounced to a greater or to a lesser extent, depending on the regions of the waveguide that are considered. Just as in the butt coupling method, the thickness of the waveguide in the passive section tapers down in order to permit the deconfinement of the optical mode, therefore the increasing of its size. The optical guide is furthermore buried in a sheathing layer.
This method has the advantage of comprising only one epitaxial step. However, it cannot be used to optimize the two waveguides, namely the active waveguide and the passive waveguide, separately. This means that it necessitates compromises. Furthermore, this method does not enable a clear definition of the boundary between the two types of guides, active and passive, because the change in state is gradual. The fact of not being able to define this boundary causes penalties because it is difficult to know where to position the electrode necessary for the operation of the component. This electrode must indeed be positioned above the active guide to ensure efficient operation of the component. By contrast, if it covers a part of the passive guide, electrical leaks are created that penalize and degrade the threshold current, efficiency current and efficiency parameters.
None of the prior art provides for an efficient mode transformation between a low index difference and a high index difference waveguide on a microchip. This invention discloses, for the first time, an efficient optical mode transformer based on a taper design, useful for transforming the mode to a high index difference waveguide on a semi-conductor microchip. The matching of the optical mode according to the invention is performed chiefly in the HC waveguide that is embedded on the semiconductor microchip.